This invention relates to shock absorbers and, more specifically, to a shock absorber assembly incorporating a liquid spring assembly capable of withstanding both compression and tension forces.
A need for highly-efficient, low cost, reliable shock absorbers has arisen in several diverse areas. Because of the increase in emphasis on automotive safety, devices have been developed for the mitigation of impact forces encountered by automobile bumpers. It has also been found desirable to provide the coupling mechanism of railroad cars with an energy absorbing system. One such device which has met with success in these applications is the liquid spring, fluid amplification shock absorber. This device employs a housing adapted to hold a body of fluid, a sweeping means, such as a piston head, slidably disposed within the housing, and a body of compressible fluid within the housing. Impact forces are transmitted through a piston rod to the piston head which then sweeps through the housing. The piston rod and head compress and displace the fluid therein, whereby the impact force is dissipated. The fluid amplified, liquid spring shock absorber is described more fully in U.S. Pat. No. 3,722,640.
While this device has been found to be highly suitable for mitigation of compressive shock forces, some difficulties have been encountered when the device is required to withstand tension loads which occur, for example, when an automotive vehicle is towed by its bumper or when railway cars are in draft.
Further difficulties have been experienced in holding the concentricity tolerances to provide even flow between the piston and housing of the aforementioned patent. Additionally, even flow problems have been encountered where the side loading of a side blow on a vehicle bumper has caused shifting of the piston head in the housing.
The incidence of malfunction of the liquid spring shock absorber associated with tension loads is related to the fact that these devices operate by virtue of a 10% compression of volume of the fluid within the housing. Piston rod displacement, therefore, dictates the size of the entire unit. In order to reduce the assembly size and weight and resulting cost, it is necessary to reduce piston rod displacement to a minimum. The piston rod is, therefore, designed to the minimum dimensions which will withstand only the compressive forces anticipated. Such design requires the use of costly, relatively brittle materials, and of course, notching or threading the piston rod in order to accomodate tension loads results in the concentration of stress forces which induces fatigue failures.
Furthermore, where the piston rod is used to accept tension loads, the point of attachment between the piston head and piston rod is also a potential stress concentration point and for this reason it has been necessary to construct a solid, one-piece piston head and rod. This, of course, is very costly due to the waste of construction materials.